We present a plan to establish an in vitro system for studying capsid assembly in the beta-herpesviruses (HCMV). Deficiencies in earlier efforts have been recognized and a strategy to overcome them is developed. There is a need for this system now to answer questions out of reach to more biochemical and biological approaches, and that need is expected to increase as more detailed information is required to develop virus- structure-based therapeutics, and as model systems for herpesvirus tegumentation and DNA packaging evolve. Our research design proceeds methodically from a series of in-cell experiments (Aim 1) that will identify the specific HCMV capsid and capsid-associated proteins required to achieve assembly in recombinant baculovirus-infected cells. As part of those studies, we will investigate the possibility that HCMV capsids require stabilization by a surface-binding protein such as the smallest capsid protein (HCMV pUL48/49), or a putative counterpart of the bacteriophage cap protein (HCMV pUL77), or perhaps the tightly capsid-associated basic phosphoprotein (HCMV pUL32), which is without a counterpart among other herpesviruses. We will also establish a cell-free in vitro capsid assembly system for HCMV (Sub-aim 1a), as a complementing and alternate approach to in-cell assembly. We will use the methods pioneered for HSV assembly to guide our work. Not only will an in vitro system substantially increase experimental flexibility, it may prove the most direct and expedient way to identify and overcome shortcomings of the in-cell approach. Recent studies demonstrate the ability of chimeric CMV scaffold proteins to drive assembly of HSV and KSHV capsid shells. In Aim 2, the effect of function-disrupting mutations in the essential HCMV UL80 scaffolding proteins (pUL80.5 and pUL80a) will be tested to assess the suitability of this method as a way to probe the molecular interactions involved in capsid formation. This system allows us to overcome barriers imposed by the essential nature of the scaffolding proteins and the inability to study these functions in HCMV infected cells. The outcome of this R21 proposal will be the development of an in-vitro capsid assembly system for a high-priority human pathogen. The future development of this system to incorporate in-vitro DNA packaging, tegumentation and envelopment of capsids will have significant impact on the development of new and novel antiviral strategies against this virus. HCMV is one of the nine herpesviruses that infect people, and prototypic of the three human beta-herpesviruses. It causes significant morbidity and mortality, especially in connection with transplacental infections and in immuno-compromised groups. New approaches are needed to prevent and treat infections by this and all herpesviruses, and the assembly system to be developed through this project will aid that endeavor.